With rapid advance of manufacturing technologies in thin film transistor LCD (TFT-LCD), and the advantages of slim and light, power saving and low radiation, LCDs have been widely used in various types of electronic products such as personal digital assistant (PDA), notebook computers, digital cameras, video cameras, mobile phones, and the like. In addition, the industry has invested heavily in research and development and uses mass production facilities. As a result, quality of LCD continuously improves and the price drops relentlessly, application scope of LCD is expanded drastically. However LCD cannot generate light by itself. It requires a backlight module to accomplish a display function.
FIG. 1 is a cross-sectional view of a conventional edge type backlight module 10. The module 10 includes a light guide plate 11, a reflecting plate 12, a lamp tube 13, a plurality of optical films 14 and a frame 15. The light guide plate 11 is made from a transparent acrylic plate which has a lower surface forming circular, hexagonal or square granular patterns by a screen printing or injection method to serve as diffusion dots 16 for scattering light. The reflecting plate 12 is located below the light guide plate 11 to reflect light emitted from the lamp tube 13 into the light guide plate 11 and to increase the utilization efficiency of the backlight module 10. The lamp tube 13 is located on one lateral side of the light guide plate 11 and is usually a cold cathode tube for projecting light into the light guide plate 11 from one end thereof. The optical films 14 are located above the light guide plate 11 and include a diffuser plate, prism sheet and polarizing sheet and the like. The frame 15 is for holding the reflecting plate 12, lamp tube 13, and light guide plate 11 and protects the backlight module 11 and the elements contained therein.
In the edge type backlight module depicted above, light emitted from the lamp tube enters the light guide plate from one end thereof and reaches the other end thereof to generate uniform lights. However, the size of the light guide plate increases with the dimension of LCD becomes larger. Light of the edge type backlight module cannot fully transmit from one end of the light guide plate to the other end due to a long path. As a result, bright variations occur from the end close to the lamp tube to the other end away from the lamp tube. Therefore, the LCD cannot generate uniform brightness.
Refer to FIG. 2 for a direct type backlight module 20. The module 20 includes a diffuser plate 21, a plurality of lamp tubes 22, a reflecting plate 23, a frame 24 and a plurality of optical films 25. The diffuser plate 21 is a white transparent acrylic plate. The diffuser plate 21 has a lower surface, a upper surface, and a plurality of diffusion dots located therein (not shown in the drawing) for diffusing lights. The lamp tubes 22 are located below the diffuser plate 21. The reflecting plate 23 is located under the lamp tubes 22. A portion of light emitted from the lamp tubes 22 directly enters the diffuser plate 21, while the rest portion of the light is reflected by the reflecting plate 23 and then enters the diffuser plate 21. The frame 24 covers the diffuser plate 21, lamp tubes 22 and reflecting plate 23 so as to protect the elements located in the backlight module 20. The optical films 25 are the same as the conventional ones depicted before, thus the details are omitted.
In the aforesaid direct backlight module 20, several lamp tubes 22 are uniformly located below the diffuser plate 21, the diffuser plate 21 may transmit uniform lights theoretically. However, due to the thickness of the diffuser plate 21, usually 2 to 4 mm, the light path is very short for lights passing through the diffuser plate 21, and lights are very difficult to be evenly diffused. As a result, some areas of the diffuser plate 21 transmit stronger scattering light beams while some other areas do not. This causes uneven brightness of the backlight module 20 as shown in FIG. 3, and a significant variation of brightness occurs on the LCD screen as shown in FIG. 4. It serious affects visual satisfactions of consumers. To remedy this problem, another conventional technique was developed. Several diffuser sheets are used to diffuse lights and obtain a more uniform light output for the backlight module. However, the improvement is limited, and each diffuser sheet has a high shielding ratio that result in a great impact to the brightness of the backlight module.
Therefore to overcome the disadvantages occurred to the conventional direct backlight modules and to develop improvement to achieve more uniform lighting to enhance the visual appealing of LCD is a goal hotly pursed in the industry at present.